A typical step-down (buck) switching regulator that regulates both output voltage and current senses a current flowing through an external resistor in order to regulate the output current flowing through a load. FIG. 1 (prior art) illustrates an exemplary prior art switching regulator 10 with a current sense resistor 11 outside of a converter integrated circuit (IC) 12. In a constant voltage mode, switching regulator 10 regulates output voltage by applying pulses of input voltage to an inductor 13 coupled to a switching bond pad 14. A first error amplifier 15 compares a first internal reference voltage VVREF to the voltage VVFB of a voltage feedback signal to create a first error voltage VVE. A controller block 16 converts the first error voltage VVE into a proportional pulse width or proportional duty cycle that is applied to inductor 13. If the magnitude of VVFB is lower than that of the reference voltage VVREF, the first error voltage VVE will increase in magnitude of the first error voltage VVE. An increasing error voltage VVE increases the pulse width or duty cycle of a top switch 17 and provides more power to the load 18. This regulation is considered constant voltage control.
In a constant current mode, switching regulator 10 regulates output current by sensing a current flowing through current sense resistor 11. Current sense resistor 11 is placed in series with load 18. The voltage drop VIFB across current sense resistor 11 is compared to a second internal reference voltage VIREF by a second error amplifier 19. Second error amplifier 19 outputs a second error voltage VIE. The first error voltage VVE is combined with the second error voltage VIE to allow switching regulator 10 to regulate either maximum output voltage or maximum output current. The two control loops enable regulator 10 to operate in both constant voltage and constant current modes.
Regulating output current using current sense resistor 11, however, has several disadvantages. First, resistor 11 is physically large, and occupies valuable space on a printed circuit board. Converter IC 12 can be made small enough to fit inside a small outline transistor (SOT) package. But current sense resistor 11 typically occupies nearly as much space as the entire SOT package. Second, resistor 11 must be precise and have a resistance that remains constant over varying temperature. The price of a precise resistor 11 can increase the cost of regulator 10 by a large portion of the cost of the entire converter IC 12. Third, current sense resistor 11 wastes power. For example, in a typical application in which a 3.6-volt battery is charging at 1 Amp, and a sense current of 1 Amp is flowing through a current sense resistor having a resistance of 100 milli-Ohms, there is a 100 milliwatt loss from the sense resistor. This represents a 2.8% loss in efficiency from the current sense resistor alone. Switching regulator 10 is less efficient and operates hotter because of the power lost through current sense resistor 11. And as the temperature of resistor 11 fluctuates over a wide range, a constant resistance is less likely to be maintained.
A switching regulator is sought that can accurately regulate output current without using a current sense resistor that is external to the converter IC of the switching regulator. In addition, a method of accurately regulating the output current of a switching regulator is sought that does not require a current to be sensed through a current sense resistor.